Specifically, the invention relates to a method for testing coins using the inductive measuring technique. This technique generally relies on the fact that a magnetic signal is directed from a transmission coil onto a coin running along a runway and a reception coil receives the resultant signal. The signal transmitted undergoes a more or less pronounced attenuation which depends on the material composition of the coin. Further, it is generally known to arrange the reception coil both on the same side as the transmission coil (DE 10 2004 013 286 B4), the entire contents of which is incorporated herein by reference, and the opposite side (DE 689 21 608 T2), the entire contents of which is incorporated herein by reference. From DE 10 2004 013 286 B4, it is also known to dispose a single reception coil on a single ferrite core and arrange the reception coil in a coaxial annular recess of the ferrite core at the front-end side facing the coin runway, the dimensions of the transmitter and reception coils being chosen so that the reception coil is permeated by a substantially homogeneous magnetic field of the transmission coil. Such an arrangement helps in testing coins at a sufficiently high resolution across the thickness of the coin with fluctuations of coin distances not having a particularly adverse effect.
For an arrangement described last, DE 198 36 490 C2, the entire contents of which is incorporated herein by reference, has made it known to provide a secondary coil next to the transmission coil that is coupled to the transmission coil. The signal of the secondary coil is applied to the second input of a differential amplifier to which the transmitted signal will get. The signal of the secondary coil is switched in such a way that the signal of the secondary coil agrees with the signal transmitted. The voltage induced in the secondary coil acts as a negative-feedback signal and causes the voltage induced in the primary coil to be equal to the voltage by which the primary coil is driven (transmitted signal). When under no load, the reception coil will thus be flown through by the same magnetic flux as is the primary coil, which causes the induced voltage in this coil to have the same curve shape as that in the transmission coil. A circuit configuration of this type, however, is also suited for an inductive measurement set-up in which the transmission coil and reception coil are disposed on different sides of the coin runway. In this case, the speech is of a transmissive measurement while a measurement on a single side only is called a reflective measurement.
The generation of a suitable transmission signal as is described in DE 198 36 490 C2 primarily is of significance for a so-called multi-frequency measurement as is described in EP 0 886 247 B1, the entire contents of which is incorporated herein by reference. In this measuring method, a periodically recurrent portion of the transmitted signal is subdivided into a number of switching steps. Envelopes are formed from the values of the signal received by the reception coil in the respective switching steps which are repeated with the frequency of the transmitted signal. An evaluation device forms at least one criterion from the number of the simultaneously generated envelopes for the generation of the acceptance or rejection signal. This measuring method is based on the findings that attenuation curves as are generated by a coin passing through the measuring device are significantly frequency-dependent. The depth of penetration is far larger in case of low frequencies than in case of high frequencies. If frequencies are very high it is known that a so-called skin effect is produced where the depth of penetration is close to zero. In the known measuring method, use is made of the property of a rectangular signal, for example, which is composed of a multiplicity of harmonics. In the portion of the rectangular signal close to its ascending edge, the signal shape of the reception coil is substantially governed by the high frequency fractions.
The signal shape is preponderantly determined by the lower frequency fractions when the number of switching steps increases.
It is also known from the initially mentioned DE 689 21 608 T2 to choose a coil assembly on the two sides in which two coils each on each side are disposed on a common core. A first transmission coil generates a signal which is received by the reception coils on either side of the coin runway. On the other side of the coin runway, the second transmission coil subsequently generates a signal which is received again by the two reception coils.
It is the object of the invention to provide a method for testing coins which allows a particularly good discrimination of most varied coin designs from forged coins.